Since IPv4 address space depletion is rapidly approaching, many Internet Service Providers (ISPs) are beginning IPv6 deployment. An IPv4 address has 32-bits, whereas an IPv6 address has 128-bits. When sending IP packets, e.g., over Ethernet, the Ethernet or link layer at the source does not “know” the hardware address of the destination. IPv4 uses Address Resolution Protocol (ARP) to allow the source to discover a destination hardware address when only its IP address is known. Once the hardware address is known, IP packets can be transmitted over Ethernet to the destination. The source stores the hardware address in a cache. Since destinations may go off line from time to time without notice, the source discards the hardware address after a period of time and sends another ARP request, if necessary.
ND is an IPv6 protocol that is similar to ARP, but provides a richer set of features. For example, in addition to address resolution, ND allows a host or source to discover attached routers or link parameters, e.g., a link Maximum Transmission Unit (MTU), through a variety of message types. ND, unlike ARP, actively maintains a list of currently bi-directionally reachable neighbors through a process known as Neighbor Unreachability Detection (NUD). NUD confirms bi-directionally reachability via positive confirmation mechanisms. In one example, NUD uses a request, e.g., a unicast Neighbor Solicitation (NS) followed by an acknowledgement, e.g., a unicast Neighbor Advertisement (NA) that confirms round trip connectivity.